Inkjet printing has developed substantially in the last decade, facilitated by the rapid advancement in both printers and substrates. On the substrate side, papers specifically adapted to inkjet printing have been developed and patented. Among the well-known technologies for paper for inkjet printing is that associated with HP ColorLok® papers. One of the critical elements in the ColorLok® technology is the incorporation of a sufficiently high local concentration of divalent ions at the surface of the paper so as to quickly immobilize pigmented printing ink. This technology is set forth in some detail in U.S. Pat. No. 6,207,258. Among the preferred divalent cations for this purpose is Ca+2.
Calcium carbonate is well-known in the industry for incorporation into paper to improve printing characteristics, but its solubility (0.0008 g/100 mL at 20° C.) requires that it be used as a suspension. Calcium carbonate is therefore commonly incorporated into the pulp slurry with the fiber. Its use in coatings that are applied to paper after the paper has been formed is complicated by the need to maintain the CaCO3 particles in suspension and by the mechanical effects (abrasion) resulting from employing particles.
Conventionally, calcium chloride has been used as the source of Ca+2 for surface treating paper because it is commercially available in bulk, and the solubility of CaCl2 in water is 74.5 g/100 mL at 20° C. Other calcium salts that are disclosed for surface treating paper include calcium acetate, calcium formate, calcium bromide and calcium nitrate. Whether these salts are in actual commercial use is less clear, but they are at least disclosed in the literature, presumably because, if one ignores economic considerations, their solubilities (34.7 g/100 mL, 16.6 g/100 mL, 143 g/100 mL and 84.5 g/100 mL at 20° C., respectively) are sufficient to contemplate their use in solution.
Although calcium chloride is by far the most commonly used calcium source for surface treatment of inkjet papers, it is not without its drawbacks. Among the more prominent of these is the corrosive nature of calcium chloride solutions. This results in considerable damage to metallic parts. It would be desirable to find a practical source of soluble Ca+2 that did not corrode metal parts in the papermaking machinery.